The treatment of contaminated soils and groundwater has gained increased attention over the past few years because of uncontrolled hazardous waste disposal sites. It is well documented that the most common means of site remediation has been excavation and landfill disposal. While these procedures remove contaminants, they are extremely costly and in some cases difficult if not impossible to perform.
More recently, research has focused on the conversion of contaminants contained in soil and groundwater based on the development of on-site and in situ treatment technologies. One such treatment has been the incineration of contaminated soils. The disadvantage of this system is in the possible formation of harmful by products including polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF).
In situ biological soil treatment and groundwater treatment is another such system that has been reviewed in recent years. So-called bioremediation systems, however, have limited utility for treating waste components that are biorefractory or toxic to microorganisms.
Such bioremediation systems were the first to investigate the practical and efficient injection of hydrogen peroxide into groundwater and/or soils. These investigations revealed that the overriding issue affecting the use of hydrogen peroxide in situ was the instability of the hydrogen peroxide downgradient from the injection point. The presence of minerals and enzymes such as catalase and peroxidase in the subsurface catalyzed the disproportionation of hydrogen peroxide near the injection point, with rapid evolution and loss of molecular oxygen, leading to the investigation of stabilizers as well as biological nutrients.
During the early biological studies from the 1980's, some investigators recognized the potential for competing reactions, such as the direct oxidation of the substrate by hydrogen peroxide. Certain researchers also hypothesized that an unwanted in situ Fenton's-like reaction under native conditions in the soil was reducing yields of oxygen through the production of hydroxyl radicals. Such a mechanism of contaminant reduction was not unexpected, since Fenton's-type systems have been used in ex situ systems to treat soil and groundwater contamination.
Other investigators concomitantly extended the use of Fenton's-type systems to the remediation of in situ soil systems. These studies attempted to correlate variable parameters such as hydrogen peroxide, iron, phosphate, pH, and temperature with the efficiency of remediation.
As with the bioremedial systems, in situ Fenton's systems were often limited by instability of the hydrogen peroxide in situ and by the lack of spatial and temporal control in the formation of the oxidizing agent (i.e. hydroxyl radical) from the hydrogen peroxide. In particular, aggressive/violent reactions often occurred at or near the point where the source of the oxidizing agent (the hydrogen peroxide) and the catalyst were injected. As a consequence, a significant amount of reagents including the source of the oxidizing agent (hydrogen peroxide) was wasted because activity was confined to a very limited area around the injection point. In addition, these in situ Fenton's systems often required the aggressive adjustment of groundwater pH to acidic conditions, which is not desirable in a minimally invasive treatment system. Finally, such systems also resulted in the mineralization of the subsurface, resulting in impermeable soil and groundwater phases due to the deleterious effects of the reagents on the subsurface soils.
U.S. Pat. No. 5,741,427 describes the complexing of a ligand donor with a metal catalyst to moderate the catalytic turnover rate of the metal catalyst. It is indicated that the preferred metal catalysts include metal salts, iron oxyhydroxides, iron chelates, manganese oxyhydroxides and combinations thereof, and the ligand donors generally comprise acids, salts of acids, and combinations thereof. The described reaction product complex of the metal catalyst and ligand donor moderates the catalytic turnover rate for a longer time and for a further distance from the injection point to provide enhanced spatial and temporal control in the formation of the oxidizing agent (i.e hydroxyl radical). Although the system described in the '427 Patent works well, the reaction product complex is highly acidic with a pH in the range of 2 to 4, which is undesirable from the standpoint of proper environmental remediation as well as regulatory review.
Other researchers have investigated the use of ozone, either alone or in combination with hydrogen peroxide, in ex situ advanced oxidation processes (AOPs). These systems suffer from a similar limitation as the ex situ Fenton's systems; namely, the necessity to pump contaminants from the in situ media to an external reaction vessel, a requirement which was both expensive and inefficient. Ozonation processes also suffer from low selectivity of contaminant destruction and high instability of the ozone and reactive species generated.
It would be of significant advantage in the art of removing contaminants from soil and/or groundwater to provide a system by which the source of the oxidizing agent and the metal catalyst can travel from the injection point throughout the aerial extent of the contamination in order to promote efficient destruction of the contaminant plume without the acidification of the subsurface or the resultant mineralization of the soils. It would be a further advantage to provide a system by which the source of the oxidizing agent is stabilized to allow dispersion throughout the plume and by which the catalytic turnover rate of the metal catalyst is moderated in order to promote more efficient destruction of contaminants. It would be of further benefit to provide an injection method in which the reagents are injected at the time, concentration, and location most suitable for efficient conversion of the contaminants at the specific site. It would be a still further benefit in the art to provide a system which efficiently generates the reactive species, for example hydoxyl radicals, to provide a cost efficient and effective method of oxidizing contaminants in soil and/or groundwater.